Alkyl methacrylates are an important class of chemical commodities which are produced worldwide on a commercial scale. Economic considerations indicate that the routes to methyl methacrylate derived from propylene have an economic advantage over synthesis methods based on isobutylene or ethylene. Methyl methacrylate currently is derived primarily from propylene via cumene formation and conversion to acetone and phenol. The acetone is then subjected to cyanation, hydrolysis and esterification steps.
A prospective alternative propylene based synthesis of methyl methacrylate proceeds via an intermediate isobutyric acid or methyl isobutyrate obtained by carbonylation of propylene. A variety of acid catalyzed (e.g., Koch reaction) and metal catalyzed carbonylation of propylene reactions are known to proceed in good yield. Oxidative dehydrogenation of isobutyric acid to methacrylic acid can be accomplished in a relatively good yield employing a heteropoly acid catalyst. However, there is no effective means for converting methyl isobutyrate directly to methyl methacrylate. In general, oxidative dehydrogenation conditions are too severe and cause hydrolysis and decarboxylation.
One method proposed for the conversion of methyl isobutyrate is by oxidation of methyl isobutyrate to the corresponding .alpha.-hydroperoxide followed by reduction of the .alpha.-hydroperoxide with propylene to provide methyl .alpha.-hydroxyisobutyrate and propylene oxide, and then a subsequent dehydration of the methyl .alpha.-hydroxyisobutyrate to yield the desired methyl methacrylate: ##STR1##
Results reported in Bull. Soc Chim. Fr., 411-4417 (1971) indicate that t-butyl .alpha.-hydroperoxyisobutyrate can produce propylene oxide from propylene in 84% yield with a molybdenum naphthenate catalyst. In addition, the dehydration of methyl .alpha.-hydroxyisobutyrate is described in U.S. Pat. No. 3,974,207.
One disadvantage of the above prospective reaction scheme for converting methyl isobutyrate to methyl methacrylate is the low reaction rate and yield obtained in the first step oxidation of methyl isobutyrate to methyl .alpha.-hydroperoxyisobutyrate. The French publication recited above reports a best yield of 13% after a 68 hour oxidation period.
Accordingly, it is an object of this invention to provide a process for converting alkyl isobutyrate to alkyl methacrylate.
It is another object of this invention to provide a process for producing methyl methacrylate and propylene oxide.
It is a further object of this invention to provide a process for converting alkyl isobutyrate to the corresponding .alpha.-hydroperoxyisobutyrate with an improved combination of conversion rate and selectivity.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.